Lithium-ion rechargeable battery cells currently use a carbon/graphite-based anode. The basic composition of a conventional lithium-ion rechargeable battery cell including a graphite-based anode electrode is shown in FIG. 1. A battery may include a single cell but may also include more than one cell.
The battery cell generally comprises a copper current collector 10 for the anode and an aluminium current collector 12 for the cathode, which are externally connectable to a load or to a recharging source as appropriate. It should be noted that the terms “anode” and “cathode” are used in the present specification as those terms are understood in the context of batteries placed across a load, i.e. the term “anode” denotes the negative pole and the term “cathode” the positive pole of the battery. A graphite-based composite anode layer 14 overlays the current collector 10 and a lithium containing metal oxide-based composite cathode layer 16 overlays the current collector 12. A porous plastic spacer or separator 20 is provided between the graphite-based composite anode layer 14 and a lithium containing metal oxide-based composite cathode layer 16: a liquid electrolyte material is dispersed within the porous plastic spacer or separator 20, the composite anode layer 14 and the composite cathode layer 16. In some cases, the porous plastic spacer or separator 20 may be replaced by a polymer electrolyte material and in such cases the polymer electrolyte material is present within both the composite anode layer 14 and the composite cathode layer 16.
When the battery cell is fully charged, lithium has been transported from the lithium containing metal oxide in the cathode via the electrolyte into the graphite-based anode where it is intercalated by reacting with the graphite to create a lithium carbon compound, typically LiC6. The graphite, being the electrochemically active material in the composite anode layer, has a maximum capacity of 372 mAh/g.
It is well known that silicon can be used instead of graphite as the active anode material (see, for example, Insertion Electrode Materials for Rechargeable Lithium Batteries, M. Winter, J. O. Besenhard, M. E. Spahr, and P. Novak in Adv. Mater. 1998, 10, No. 10). It is generally believed that silicon, when used as an active anode material in a lithium-ion rechargeable cell, can provide a significantly higher capacity than the currently used graphite. Silicon, when converted to the compound Li21Si5 by reaction with lithium in an electrochemical cell, has a theoretical maximum capacity of 4,200 mAh/g, considerably higher than the maximum capacity for graphite. Thus, if graphite can be replaced by silicon in a lithium rechargeable battery, a substantial increase in stored energy per unit mass and per unit volume can be achieved.
In lithium-ion rechargeable battery cells using a graphite-based anode, the graphite is in the form of a fine powder whose particles are held together by a binder. Polyvinylidene fluoride (PVDF) and styrene butadiene rubber (SBR) are the most commonly used binders in graphite anodes but other binders have been suggested, for example U.S. Pat. No. 5,660,948 discloses the following binders in a carbon anode of a lithium ion cell: ethylene-propylenediene termonomer, PVDF, ethylene-acrylic acid copolymer and ethylene vinyl acetate copolymer.
U.S. Pat. No. 6,399,246 teaches that poly(acrylic acid) does not provide good adhesive properties in graphite anodes of lithium-ion battery cells and claims the use of a polyacrylamide binder.
U.S. Pat. No. 6,620,547 discloses a lithium secondary cell having a carbon anode, in which lithium may be intercalated, and a cathode formed from a transition metal held by a matrix polymer. The polymer used has an affinity for the transition metal ions so that they are held on the polymer chains. The polymer may be selected from a number of materials such as polyacrylate, poly(acrylic acid), polymethylmethacrylate, poly(vinyl pyrrolidone), polyacrylonitrile, poly(vinylidene fluoride) and poly(vinyl chloride).
U.S. Pat. No. 5,260,148 discloses a lithium secondary cell having an anode formed from a lithium compound that is held together by a binder, which may be starch, carboxymethyl cellulose (CMC), diacetyl cellulose, hydroxypropyl cellulose, ethylene glycol, poly(acrylic acid), polytetrafluoroethylene and poly(vinylidene fluoride).
The most common binders used in graphite anodes of lithium ion cells (PVDF and SBR) do not bind silicon electrode material cohesively together in silicon-based anodes over successive charging cycles and it is believed that this is due to the relatively large volume changes associated with the insertion and removal of lithium ions into the silicon material during the charging and discharging stages of the cells. The volume changes are much larger than in the corresponding graphite anodes and can result in individual silicon particles not always re-establishing electrical contact with each other and with a current collector when the silicon anode shrinks due to the removal of lithium ions during discharging.
An alternative binder that has been proposed for silicon systems is sodium carboxymethylcellulose (NaCMC) Na-CMC adequately functions as a binder when used in conjunction with high purity silicon, of the type used to fabricate integrated circuit (IC) Si-wafers. However, such silicon is very expensive. When using relatively cheap, lower-grade silicon, there are minor amounts of impurities present that are not chemically compatible with the binder solution and that cause a decrease in the viscosity of the silicon/binder mixture. As a consequence, the resulting coating does not retain sufficient contact with the current collector so as to undergo anything more than a limited number of discharge/recharge cycles, before losing its capacity to hold a charge.
Journal of Applied Electrochemistry (2006) 36:1099-1104 discloses the use of an acrylic adhesive as a binder for the anode of Li-ion batteries. The anode material is a Si/C composite so has a lower volume change than electrodes where the anode is Si alone. There is no disclosure of the nature of the acrylic adhesive other than a reference to product LA132 whose composition is believed to be a mixture of acrylonitrile and butadiene in methylethyl ketone, ethyl acetate and toluene.
J Power Sources, 161 (2006), 612-616 describes a carbon anode of a lithium ion battery, which also contains NaCMC as a thickening agent and SBR as the binder. PAA (poly(acrylic acid)) is added as a surface active dispersing agent.
J Power Sources, 173 (2007), 518-521 addresses a problem of graphite electrodes for Li-ion cells when using propylene carbonate solvent/electrolyte since the propylene carbonate is intercalcated into the graphite electrode during charge/discharge, causing solvent decomposition and graphite exfoliation. The addition of PAA solves this problem.